1. Submission doc.: IEEE 802.11-15/0823r1 July 2015 Sungho Moon, NewracomSlide 1 Preamble Design and Auto-Detection for 11ax Date: 2015-07-13 Authors:

2. Submission doc.: IEEE 802.11-15/0823r1 July 2015 Sungho Moon, NewracomSlide 2 Abstract In the same platform, the previously proposed repeated L-SIG[1] and signature symbol schemes[2] are evaluated The repeated L-SIG scheme needs optimization efforts for repetition threshold considering a trade-off between false detection and mis-detection probabilities The signature symbol scheme shows reasonable performance in both mis-detection and false detection For a simple implementation and future extension, the signature symbol scheme is more preferred than the repeated L-SIG scheme

3. Submission doc.: IEEE 802.11-15/0823r1 Introduction Repeated L-SIG (RL-SIG) [1] Modulating the RL-SIG (L-SIG repetition ) symbol with BPSK and rate ½ BCC. Detection from both a repetition check and an L-SIG validity check Signature symbol (SS) [2] One symbol, MCS 0, separately encoded Signature of 10~12 fixed bits Additional info. of 6~8 bits Detect from checking a known signature after decoding Slide 3Sungho Moon, Newracom July 2015 L-SIG 4us HE- SIGA R-LSIG 4us BPSK L-SIG 4us HE- SIGA Signature 4us BPSK … …

4. Submission doc.: IEEE 802.11-15/0823r1 Simulation Environments Bandwidth : 20MHz Multi-antenna transmission with CSD: 1x1, 2x1, and 4x1 Wireless channel: TGac D and UMi Carrier frequency offset (CFO): fixed at 40 ppm (@ 5GHz) Phase noise (both at Tx/Rx): -41dBc Real timing estimation & synchronization Signature symbol configuration [2] 12 bits for signature, 6 bits for tail, and 6 bits for random information 11ax detection algorithms Explain in the following pages SIG-A assumption: 34 payload + 6 tail + 8 CRC bits (2 OFDM symbol) Slide 4Sungho Moon, Newracom July 2015

5. Submission doc.: IEEE 802.11-15/0823r1 Detection Algorithm for 11ax : Repeated L-SIG (RL-SIG) Slide 5Sungho Moon, Newracom July 2015 Timing/CFO compensation Equalization L-STF L-LTF Repetition Threshold > α Legacy Detection MRC & L-SIG Validity Check Y N N t L-SIG RL-SIG Y 11ax detect11n11ac11a The same detection algorithm in [1] Repetition threshold, α Cross-correlation value btw. L-SIG and RL-SIG L-SIG validity check Parity = OK L-Rate = 6Mbps L-Length (mod 3) = 0

6. Submission doc.: IEEE 802.11-15/0823r1 Detection Algorithm for 11ax : Signature Symbol (SS) The same detection algorithm in [2] Signature check After decoding with the tail bits, the 12 bits are matched with the known signature Slide 6Sungho Moon, Newracom July 2015 Timing/CFO compensation Equalization L-STF L-LTF Signature Check Legacy Detection Y N t L-SIG 11ax detect 11n11ac11a SIGNATURE

7. Submission doc.: IEEE 802.11-15/0823r1 Mis-Detection & False Detection Mis-detection in the 11ax receiver When an 11ax PPDU is transmitted, an 11ax device detects it as other types of PPDUs Two types of false detections Type 1 (to see impacts to legacy devices): When an 11ax PPDU is transmitted, a probability that an 11ac (or 11n) device detects it as an 11ac (or 11n) PPDU It should be checked if a new 11ax PPDU has unusual modulations in the position of 11n/11ac SIG-A symbols Type 2 (to see impacts from legacy PPDUs): When an 11ac (or 11n or 11a) PPDU is transmitted, a probability that an 11ax device detects it as an 11ax PPDU In this contribution, the type 2 false detection is considered. Type 1 false detection has minimal system impact Slide 7Sungho Moon, Newracom July 2015

8. Submission doc.: IEEE 802.11-15/0823r1 Mis-Detection Performance The RL-SIG shows 1.0~1.5 dB gain compared to the SS scheme due to MRC combining of two L-SIG symbols The both schemes shows similar mis-detection curves to each of L-SIG errors Slide 8Sungho Moon, Newracom July 2015 L-SIG and mis- detection of SS L-SIG and mis- detection of RL-SIG 1.0 dB Both schemes show error floors in UMi 1.5 dB

9. Submission doc.: IEEE 802.11-15/0823r1 Mis-Detection Performance (cont’d) Both schemes show no serious degradation or other noticeable aspects in multi-antenna transmissions Compared to 1x1 in TGac D, the 2x1 has approximately 1.0 dB gain @ 10 -1 Compared to 1x1 in UMi, the 4x1 has approximately 1.7dB gain @ 10 -1 Slide 9Sungho Moon, Newracom July 2015 1.0 dB1.7 dB

10. Submission doc.: IEEE 802.11-15/0823r1 False Detection for RL-SIG The false detection increases as SNR increases for 11ac/11a PPDUs Even at a high SNR, over 4% of 11ac PPDUs are detected as 11ax PPDU due to the high false detection The same trend is verified in AWGN (Appendix A) Slide 10Sungho Moon, Newracom July 2015 Most of 11n PPDUs can be filtered out in the repetition check since it has QBPSK symbol L-SIG (BPSK) SIG-A1 (BPSK) 11ac PPDU L-SIG (BPSK) SIG-A1 (QBPSK) 11n PPDU L-SIG (BPSK) Data (QAM) 11a PPDU 11ax Receiver Falsely Detected as 11ax Correctly Detected as others … … … … … … About 4% false detection

11. Submission doc.: IEEE 802.11-15/0823r1 False Detection for RL-SIG (cont’d) In high SNR, 11ac PPDUs are falsely detected as 11ax L-SIG validity check does not work properly in high SNR HE STA combines 11ac L-SIG and VHT-SIG-A1 (in MRC) for decoding If cross-correlation is high enough (according to our simulations, above 0), combined L-SIG + VHT-SIG-A1 successfully decodes as L-SIG. VHT-SIG-A1 is not trellis terminated and acts as interference to L-SIG. If combined second OFDM symbol (e.g. VHT-SIG-A1) is self-decodable (i.e. trellis terminated), the combined signal can be decoded either as L-SIG or the second OFDM symbol. (Appendix B) L-SIG at 0dB (AWGN) can be decoded with 99.7% probability (Appendix C) Slide 11Sungho Moon, Newracom July 2015

12. Submission doc.: IEEE 802.11-15/0823r1 False Detection for RL-SIG (cont’d) False detection and mis-detection probabilities trade-off With a large repetition threshold α (= tight repetition check), the false detection is reduced But the mis-detection increases (more 11ax PPDUs are filtered out in the repetition check stage) Slide 12Sungho Moon, Newracom July 2015 False detection decreases with α Mis-detection increases with α Mis-detection is worse than Non-Combined L-SIG PER

13. Submission doc.: IEEE 802.11-15/0823r1 False Detection in the Signature Symbol Good false detection probabilities in both indoor and outdoor channels Always lower than 10 -3 (regardless of SNR and PPDU types) False detection that also checks SIG-A CRC is below 10 -4 Slide 13Sungho Moon, Newracom July 2015 Not seen above 10 -4 when SIG-A CRC is checked

14. Submission doc.: IEEE 802.11-15/0823r1 Potential Issues in the RL-SIG The false detection probability increases with SNR Worst case: 11ac PPDU or 11a PPDU with BPSK data (e.g. management or control packet) False detection results in loss of 11ac or 11a packet entirely False detection can be mitigated with HE-SIG-A CRC check Results in more complex receiver architecture (due to potential 11n/11ac AGC symbol) Benefits of early detection (right after L-SIG) lost Complex receiver architecture & optimization In order to get any MRC gains (from duplication), complex adaptive cross- correlation detection algorithms is needed. Implementation margin is likely to eat up any MRC gain. Robustness of the adaptive cross-correlation detection algorithm is questionable. Detection algorithm must take into account channel characteristics, SNR, potential PPDU types, etc. Slide 14Sungho Moon, Newracom July 2015

15. Submission doc.: IEEE 802.11-15/0823r1 Conclusion Repeated L-SIG scheme, has high false detection probability for 11ac PPDUs and 11a BPSK PPDUs. Requires complex receiver architecture to cope with false detection issues. 1 dB MRC gain of L-SIG is washed away when taking into account false detection issues. With wrong parameter configuration, even worst performance than single L-SIG decoding Future extension of PPDU formats is important and should be addressed Extension of repeated L-SIG will be limited and may cause even more miss- detection/false detection issues. Signature symbol scheme is preferred Simple implementation (no additional optimization needed) Robust performance under any scenario Great future extension ability (additional 6~8 bits for 11ax and future use) Slide 15Sungho Moon, Newracom July 2015

16. Submission doc.: IEEE 802.11-15/0823r1 Straw Poll Do you agree that auto-detection design (e.g. HE PPDU preamble design) shall take into account mis- and false detection probabilities together with optimization complexity in the implementation? Slide 16Sungho Moon, Newracom July 2015

17. Submission doc.: IEEE 802.11-15/0823r1July 2015 Sungho Moon, NewracomSlide 17 References [1] 11-15-0579r2, Preamble Design and Autodetection [2] 11-15-0643r0, Autodetection with Signature Symbol

18. Submission doc.: IEEE 802.11-15/0823r1 Appendix A: Verification in AWGN Slide 18Sungho Moon, Newracom July 2015 Repetition Threshold > α MRC & L-SIG Validity Check Y Y N N L-SIG(1:24)SIG-A(1:48) Encoding AWGN (1:48 ) A B C False detection prob. (= C/A) As SNR increases, the increase in the false detection can be seen as well in AWGN This increase comes from the L-SIG validity check (See the ratio C/B the next page) Simple bit-level realization of 11ac PPDUs take the first 48 modulated symbol Combine two symbols

19. Submission doc.: IEEE 802.11-15/0823r1 Appendix A: Verification in AWGN (cont’d) Validity check pass ratio = C/B For all SNRs, it has over 80% pass ratio and increases with an increase in α value Slide 19Sungho Moon, Newracom July 2015 Repetition check pass ratio = B/A It is mostly independent to SNR and varies significantly with α value

20. Submission doc.: IEEE 802.11-15/0823r1 Appendix B: Effect from SIG-A Encoding Assuming the interfered symbol (SIG-A1) is a self-decodable (i.e. trellis terminated within the symbol) (Blue curve), With some chances, the decoding Trellis of the combined signal (L-SIG + SIG-A1) can follow SIG-A1’s because it is also self-decodable However, the current 11a/11ac/11n SIG-A1 is a portion of longer encoded information (Red curve), SIG-A1 is not self-decodable Therefore, highly likely to be decoded as L-SIG and pass the L-SIG validity check Therefore, the L-SIG content check of the combined L-SIG is not useful Slide 20Sungho Moon, Newracom July 2015 50% chance of L-SIG validity check pass

21. Submission doc.: IEEE 802.11-15/0823r1 Appendix C: L-SIG PER in AWGN Approximately 99.7% of L-SIG symbols can be decoded correctly even at 0 dB The 0 dB is almost equivalent to the condition combining an L-SIG symbol with the same powered random symbol without noise Slide 21Sungho Moon, Newracom July 2015

22. Submission doc.: IEEE 802.11-15/0823r1 Appendix D: Repetition Check (1/2) Slide 22Sungho Moon, Newracom July 2015 Note: Hamming distance of 8 corresponds to -0.67 normalized cross correlation

23. Submission doc.: IEEE 802.11-15/0823r1 Appendix D: Repetition Check (2/2) Slide 23Sungho Moon, Newracom July 2015